Randomized, double-blind, placebo-controlled, multicenter, parallel group comparison study designed to evaluate the efficacy of chronic, passive antibody immunotherapy employing one of two doses of a mucoid exopolysaccharide Pseudomonas aeruginosa immune globulin, intravenous (human) (MEP IGIV) in cystic fibrosis (CF) patients with endobronchial colonization by mucoid Pseudomonas aeruginosa. Specific aims are to assess the efficacy of monthly MEP IVIG in reducing the frequency of acute pulmonary exacerbations over 1 year in patients with mild to moderate pulmonary disease, and to obtain population based MEP IVIG pharmacokinetics during chronic therapy and to assess the effect of therapy on the rate of decline of FEV1 and sputum density of mucoid P. aeruginosa.